Use cargo rustc to build Fornjot models

This provides more flexibility and the potential for making it easier
for the user to specify models.

crates/fj-host/src/lib.rs

@@ -90,7 +90,7 @@ impl Model {
         let mut command_root = Command::new("cargo");
 
         let command = command_root
-            .arg("build")
+            .arg("rustc")
             .args(["--manifest-path", &manifest_path]);
 
         let cargo_output = command.output()?;